Hmmm. I'm still debating about books 1 and 2 (both of which have been on my TBR since forever). I think after this review I'll wait to see what you're going to make of book 4 before taking a step closer towards starting the series!
If they're already on your TBR, I'd go ahead and read them - they're interesting, though I'm going to fall on the side of them never being favourites of yours. Unless you're a completist, in which case, let's see if a fourth book is going to be published and see what it's like. This one was good, just over played a bit in my opinion.
Well, they're not on my physical TBR yet ... :D (Which is kind of why I'm holding back. I'm hearing a lot of praise for the series, but I think you might be right -- there's some vibe I'm getting from the blurbs and from everything I'm reading about the books that makes me a bit cautious about them.)
Ah, well then ... maybe if you run out of historical fiction (HA!) and you're near a library ... give them a try? They're not total fluff or anything, and I don't think you'd hate them, just maybe find them meh-ish. (though I could be totally wrong - I'm a sucky judge of taste.)